1. Field of the Invention
The present invention is directed generally to data storage in a computerized data processing system, and more particularly, to the recording of archive storage volumes using optical media in a cost efficient manner designed to maximize tamper resistance and ensure a high degree of certifiability.
2. Description of the Related Art
A common practice in computer data processing environments (from very small home computers to very large enterprise computers) is to store data sets (e.g. data and program files) onto removable optical media, including disks recorded using rewritable and Write-Once Read Many (WORM) techniques. Usually, these data sets are copied or moved to the optical media from a direct access storage device (DASD), such as a disk drive, as part of a migration, backup or archive operation.
Rewritable optical disks are used in data processing enterprises for space management and data backup operations. Space management includes data migration, which is the act of moving infrequently used data sets from primary storage to migration storage. Backing up is the act of periodically copying data sets, or portions thereof, from primary storage to backup storage in order to create one or more backup versions of the data sets which can be recovered following a disaster event. Rewritable optical media are used for migration and backup because the data sets recorded thereon usually become obsolete, and the migration and backup disks can be reused to record new migration and backup data. The prevailing rewritable optical disk technology used today is Magneto-Optical (MO) recording. MO disks have surfaces with fixed magnetic domains therein that change the polarization direction of an incident laser beam to produce a modulated return beam containing the information recorded on the media. When it is desired to write new information to the disk, the laser power is increased so as to heat the media to a point where the magnetic domains can be altered by an external magnetic field, but not so high as to ablate the media.
WORM disks are used in data processing enterprises for data archival. Archival is the act of saving a specific version of a data set (e.g., for record retention purposes) for an extended period of time. The data set is placed in archive storage pursuant to command by a user or data processing administrator. Archived data sets are often preserved for legal purposes or for other reasons of importance to the data processing enterprise. It is therefore important that archived data volumes be capable of certification, meaning that automatic machine procedures are in place for certifying that the data sets written to the archive volume have not been altered or rewritten. Disks recorded according to WORM techniques, are often used for archival purposes because they can be written only once. There are two distinct methods being offered in the marketplace for WORM recording: WORM using ablative media, and Continuous Composite Write-once (CCW) using magneto-optical media.
Ablative WORM disks are recorded using a high power laser beam which permanently ablates the media to form small pits which alter the reflectance of the media surface. When an incident laser beam (at a lower power level during read mode than during write operations) is focused on the media, there is produced an intensity modulated return beam containing the information recorded on the media. Ablative WORM thus provides a permanent audit trail of archived data based on the ablative nature of the recording media. In contrast to Continuous Composite Write-once (CCW) uses a magneto-optical media and an optical data storage device that allows the media to be convertible from re-writable to read-only using drive firmware. Each media recording surface has a media descriptor table contained within a control track which defines the media as a unique media type. Previously manufactured drives will not recognize the media type, and therefore, will not read or write the media. The data on the media is therefore protected from being destroyed by such drives. There is also a storage state indicator within each sector of each track of the media that defines whether the sector is writable or read-only. When the indicator is in the "off" state the sector may be written. The writing process changes the state of the indicator to "on" or "read only," which prevents any further writing on the sector.
Ablative WORM technology has been successfully marketed as superior to CCW technology due to the built-in tamper-resistant protection of the ablative media versus the perceived tamperable protection offered by CCW drive firmware. However, the use of ablative technology has disadvantages with respect to the development time, development expense, and unit cost required for the drive and the media.
Accordingly, a superior method is required for WORM data storage. What is required is a storage methodology that reduces the substantial costs of ablative WORM yet provides greatly improved tamper resistance over CCW technology.